I agree that +Buy and +$ are great, but the lack of those wouldn't have ruined my enjoyment of the game. Additional cards and actions are kind of key in this game, and I probably would have lost interest if I didn't have those.
If there were never attacks or other interaction, then I don't think we'd have noticed. It definitely would have added more weight to the criticism that Dominion is just multiplayer solitaire, but I don't think it would diminish the game.
Trashing is interesting. It is such an integral part of the game that when I play other deckbuilders, like Legendary, one of the first thing I do is identify any way to remove chaff and streamline my deck. But you know, if Dominion never had any trashers, I don't think I would have even noticed it wasn't there. I'd continue playing with my 7 Coppers and 3 Estates and base my strategy around them being there. Eventually, a future expansion or some other deckbuilding game would introduce it, and it would have been quite the "ah-ha" moment.